Sheet medium processing device, such as printer, scanner and currency validators, performs processing such as printing, scanning and identification when a medium moves along a medium conveying passage. In order to adapt to as many medium types as possible, the width of the medium conveying passage is generally set according to the maximum width of the medium. For example, the printer used in a bank needs to print various types of mediums such as bank book, deposit receipt and transaction log, in which the width of the bank book is the smallest and the width of the medium for printing the transaction log is the biggest, wherein the width of the medium for printing the transaction log is at least twice the width of the bankbook. In order to satisfy the printing of the bankbook, the deposit receipt and the transaction log simultaneously, the width of the medium conveying passage of the printer is adapted to the width of the medium for printing the transaction log. Since the width of the medium conveying passage is far greater than the minimum width of medium, it is needed to arrange a medium detection device in the medium conveying passage to detect the position and state of the medium in the medium conveying passage.
There are two types of common medium detection mechanisms in the art.
The first type is to arrange a set of light detector at a fixed position of the medium conveying passage. Generally, the light detector comprises a light receiver and a light emitter which are provided on two sides of the medium conveying passage correspondingly; and medium can pass through between the light receiver and the light emitter. If there is no medium passing between the light receiver and the light emitter, the light beam emitted by the light emitter almost is fully received by the light receiver, then the light receiver outputs a first electrical signal. If there is a medium passing through between the light receiver and the light emitter, the medium blocks the light beam emitted from the light emitter to the light receiver so that the light receiver can not receive the light beam, then the light receiver outputs a second electrical signal. Thus, by detecting the change of the electrical signal output by the light receiver, it can be judged whether there is a medium passing through the fixed position. The defect of this scheme lies in that the medium must be input to the medium conveying passage from a position provided with the medium detection device; otherwise, the medium can not be detected; therefore, the flexibility of operation is low.
The other type of medium detection device is as shown in FIG. 1; multiple sets of light detectors are aligned and provided in the medium conveying passage perpendicular to a medium conveying direction. Each set of light detector comprises a light receiver 10 and a light emitter 10′ which are provided oppositely with a preset distance, and the medium can pass through between the light receiver 10 and the light emitter 10′. Similar to the first type of medium detection device, the position and the state of the medium are determined by detecting the change of the electrical signal output by the light receiver. The advantage of this scheme lies in that the medium detection device can detect the position and the state of medium no matter where the medium is input from in the passage; therefore, the operation is relatively flexible. However, since the light emitter is a key part of the medium detection device and this scheme needs to arrange a plurality of light emitters, the cost of the device is high.